1. Field of the Invention
The invention relates generally to liquid nitrogen handling equipment and more specifically to funnels with vacuum-insulated walls and float valves for filling small atmospheric pressure dewars of liquid nitrogen used, for example, by high-performance auxiliary equipment on scanning electron microscopes that are generally referred to as energy dispersive x-ray (EDX) accessories.
2. Description of the Prior Art
When liquid nitrogen is held in a container with any kind of pressure, pouring or piping it out into an atmospheric pressure vessel becomes a great problem because the liquid almost unavoidably changes phase to a gas and liquid mixture that is difficult to deal with. The handling of liquid nitrogen is a common chore in laboratories equipped with scanning electron microscopes (SEMs). Reservoirs of liquid nitrogen, commonly called dewars", are held high above the laboratory floor on top of the EDXs to help chill the sensitive electronic detectors. Such dewars have insulated walls and corks of foam that are used as stoppers. Because it is critical that such reservoirs never go empty, a technician must routinely climb a step ladder, pull out the stopper and pour in the liquid nitrogen until the dewar is full. The stopper must be replaced immediately, or chilled air will form water crystals of ice around and inside the top of the dewar and contaminate the liquid nitrogen supply inside the dewar.
SEMs are extremely sensitive to vibrations since they multiply images by as much as one hundred thousand times and are often used to scan features smaller than one micron. Therefore, SEMs are conventionally isolated from general building vibrations by locating them completely on a special vibration-free table. Piping and plumbing systems for pumping liquid nitrogen directly to an SEM from an outside tank suffer from the same outside building vibrations. Any physical connection of such plumbing systems to the SEM EDX dewar can transfer unwanted vibrations to the SEM. Therefore, conventional liquid nitrogen supply systems do not plumb the liquid nitrogen from an outside source to the SEM. Hand-held flasks have been universally used to manually fill SEM EDX dewars with standard funnels. Because of their conventional construction, ordinary funnels must be removed immediately after use.